The most conventional system design in a microwave system includes an indoor unit and an outdoor unit mounted separately from each other and connected via an intermediate frequency (IF) cable to each other. Signals transmitted over the cable include a transmission/reception IF signal, a transmission/reception control signal and a 48 V power signal. Refer to FIG. 1, which shows the operational principle of the prior art. As shown in FIG. 1, in the transmission direction, after the transmission data is modulated by a modulation/demodulation unit 10 in the indoor unit 1, the data is transmitted to a first IF transmission unit 11 to be mixed, filtered and amplified, is then sent via an IF cable to a combining/branching unit 13 of the outdoor unit 2 for branching after having been combined by a combining/branching unit 12 in the indoor unit 1, is subsequently processed by a second IF transmission unit 14 for cable compensation, filtering, amplification, mixing and filtering process, undergoes mixing, filtering and amplification by a microwave transmission unit 15, and is finally transmitted to an antenna via a duplexer 16. In the reception direction, the signal received by the antenna is amplified, mixed and filtered by a microwave reception unit 17 of the outdoor unit 2, is then sent to a second IF reception unit 18 for amplification, mixing, filtering and amplification, and is subsequently sent via the combining/branching unit 13 to the indoor unit 1, which separates the received signal via the combining/branching unit 12; the separated signal undergoes automatic gain control, filtering and sampling by a first IF reception unit 19, and is finally sent to the modulation/demodulation unit 10 to be demodulated into a traffic data signal.
The signal bandwidth of the IF transmission/reception signal is markedly increased in application to large capacity microwave, and in practical application the transmission signal occupies even more signal bandwidth during predistortion. Moreover, in order to ensure IF transmission/reception interval, the transmission/reception frequency may have to be markedly increased, while increase of the frequency means more attenuation of the cable, so that the prior art separate design of the indoor unit and the outdoor unit in application to such large capacity microwave not only increases the difficulty in design, but also inevitably requires less attenuation of the IF cable due to more attenuation of the signal caused by increase in the IF. Consequently, stricter requirement is called for the cable, and the production cost of the cable is hence increased.